1. Field of the Invention
The present invention relates in general to a hydraulically operated braking system for braking a drive wheel or wheels of a motor vehicle, and more particularly to a control device for controlling the braking system, so as to maintain amounts of slip of the drive wheels within an optimum range, during brake application or acceleration of the vehicle.
b 2. Discussion of the Prior Art
When a brake is applied to a motor vehicle, the wheels of the vehicle will slip on the road surface if braking forces applied to the wheels exceed friction forces between the wheels and the road surface. The required braking distance of the vehicle increases with an increase in the amount of slip of the wheels. In light of this fact, there is proposed an anti-skid braking system wherein the fluid pressure to be applied to the wheel brake is automatically adjusted within an optimum range, as disclosed in the Manual for New "TOYOTA SOARER" (published on Jan. 21, 1986, by Toyota Jidosha Kabushiki Kaisha, the assignee of the present application).
The vehicle wheels will also slip on the road surface if drive forces transmitted to the drive wheels exceed the friction forces between the wheels and the road surface, upon rapid acceleration of the vehicle, for example, upon starting of the vehicle. In this case, the slip of the drive wheels results in lowering the acceleration rate of the vehicle or reducing the running stability. In view of this phenomenon, there is proposed a braking system incorporating a so-called "traction control" device which is adapted to automatically activate the brakes for preventing excessive amounts of slip of the drive wheels, by absorbing an excess of the drive force exerted on each drive wheel, as disclosed in Laid-Open Publication No. 58-16948 (published in 1983) of unexamined Japanese Patent Application No. 57-119196.
The proposed braking systems indicated above use a brake control arrangement which includes slip-detecting means for detecting the amount of slip of the wheels, and pressure-regulating control means for regulating the brake fluid pressures to be applied to the wheel brakes. The slip-detecting means produces output signals indicative of the detected amounts of slip of the wheels, and the pressure-regulating control means is operated in response to the output signals from the slip-detecting means, for increasing or decreasing the brake fluid pressures so as to maintain the amounts of slip of the drive wheels of the vehicle within a suitably selected range.
In the case where the anti-skid brake control or traction control is applied to the drive wheels, the pressure control system may suffer from cyclic control vibrations, due to a resonance of the drive system that drives the drive wheels, whereby the pressure-regulating control means fails to normally function to keep the amounts of slip of the drive wheels within the desired or optimum range. Stated more particularly, the engine, power transmission units, and drive wheels constitute a spring mass system which is likely to vibrate at the natural frequency. As a result, the rotating speeds of the drive wheels tend to change, without dependence upon the magnitude of the braking forces applied to the drive wheels, as compared with the reaction forces of the road surface. The pressure-regulating means is operated to control the brake fluid pressures for the drive wheels, based on the drive wheel speeds which change as described above. Accordingly, the brake pressure regulation cannot be adequately effected, leading to insufficient or excessive brake pressures applied to the drive wheels. In other words, the slip amounts deviate from the optimum range. In some instances, the brake pressure regulation by the pressure-regulating control means may even promote the resonance tendency of the drive system.